The present invention concerns an optical element that comprises a base body and an optically effective layer system of which at least one layer system surface contacts the base body.
The invention also comprises a related manufacturing process, the utilization of such a component or process, as well as an optical projection arrangement.
Introduction:
German patent reference DE-40 33 842 discloses a cuboid optical element composed of dichroitic layers referred to as a “dichroitic prism.”
In the present application the term X-cube is used.
The present invention starts with the problems that exist with known X-cubes, as described, for example, in DE-40 33 842, or that occur in its manufacture. The present invention which was developed in order to find a solution to the problems with such elements, can be applied to a number of other optical elements.
For this reason this description begins with the specific problems to be solved on X-cubes and based thereon explains the application of the invention in more general terms.
Description:
Based on FIG. 1, the basic functional principle of an X-cube is explained. Optical elements of this type are principally used in projectors in order to recombine the red, green and blue channels in the spectral range of visible light. As shown in FIG. 1 such an X-cube comprises four individual prisms 2a to 2d which can, for example, be made of BK7 glass. In their cross-section they form right-angled isoceles triangles with an angle of 90 degrees, usually with a tolerance of over ±60 angular seconds, and hypotenuse angles of 45 degrees with tolerances of a few angular minutes. The length of the hypotenuse is typically between 5 mm and 50 mm, preferably 40 mm. Embedded between the two prism pairs 2a and 2b on the one side, and 2d and 2c on the other side, there is an optically effective layer system 5 that largely reflects visible light in the blue range but largely transmits visible light in the green or red range. FIG. 1 shows a part of the blue reflector layer system as a color splitting system, labeled 5′, the other 5″.
Embedded between the two prism pairs 2a and 2d on the one side, 2b and 2c on the other side there is an additional, optically effective layer system 7, that largely reflects light in the red range but largely transmits light in the green range and the blue range. In FIG. 1 also the two legs of the red reflector layer system are shown as a color splitter system, labeled 7′ and 7″.
On the X-cube there are three input channels for red, green and blue light from corresponding sources, for example, LCD controlled, and an output channel with the recombined input signals. On the reflector systems, between each of said prism pairs, the correspondingly colored light, and particularly S-polarized light with an incidence angle of less than 45 degrees is reflected. In addition the hypotenuse surfaces of the prisms 2 can be and usually are coated with an antireflection layer system.
Because the pixels of the red-blue-green input channels should converge as accurately as possible, the angle tolerances on prism 2 and in the assembled X-cube must be very narrow.
Large tolerances result in poor imaging quality because the pictures do not accurately converge: blurring or color fringes occur.
Location 9, shown with dashes in FIG. 1 where the four individual prisms 2 meet, is also located within the imaging optical path. Optical interferences created in this location manifest themselves, as mentioned for example, as a blurred picture in the output channel OUT. It is a requirement of such elements and their manufacturing process to minimize the interferences, particularly in this location 9.
From DE-40 33 842, for example, it is known that X-cubes can be manufactured from four prisms 2 according to FIG. 1. The four individual prisms are first manufactured in their exact dimensions through milling, grinding and polishing. Subsequently they are coated with the appropriate layer system along their leg and possibly on their hypotenuse surfaces with an antireflection coating. Finally the coated individual prisms 2 are cemented together.
Disadvantages of the Known Processes and Known X-Cubes:
The handling effort required for manufacturing the X-cubes as described, for example, in DE-40 33 842, is very high: First, each of the three lateral sides of each individual prism 2 must be mounted or fixed by plastering, blocking or wringing as shown in FIG. 1 before the glass can be worked. Subsequently the surfaces must be cleaned for coating the individual prisms 2, and then mounted and dismounted for the coating process. On an average two sides per individual prism need to be coated. This laborious handling considerably raises the production costs for such X-cubes.
From FIG. 1, particularly location 9, it is evident that the coating of the red and blue eflecting layer systems must be executed in such a way that the coating does not wrap around the 90 degree edges of the individual prisms. This requires sophisticated coating fixtures or masking of the legs on which no coating may be deposited. In this respect we refer to U.S. Pat. No. 2,737,076 (Rock et al.).
During the coating and the handling of the individual prisms 2, the 90 degree prism edges are exposed without protection, that is, especially those edges which according to the foregoing explanation must be very accurate. This inevitably leads to chipping unless laborious precautions to protect these edges are taken, which again increases the costs.
If, for example, anything goes wrong during the coating of the individual prisms 2, such an individual piece must be remounted, ground, and repolished, otherwise it will have to be discarded. Correction processes are at best very difficult to implement.
Cementing of the individual prisms 2 in the exact relative position is very difficult and laborious. Complicated processes such as described in DE-40 33 842 are required. Prisms are cemented individually which is time-consuming and therefore costly.
Independently of said disadvantages the known process results in a structure in location 9 as shown in FIG. 1, as can be seen from the detail in FIG. 2. The same reference marks are used as in FIG. 1 are used. Item number 11 identifies the cemented joints.
From this it is evident that the cemented joints 11 cause an interruption of the red light reflection layer system 7 (consisting of 7′ and 7″) as well as the blue light reflection layer system 5 (consisting of 5′ and 5″).
As the X-cube is manufactured by cementing the individual prisms 2 whose 90 degree edges have been exposed to external influences without protection, faults occur almost inevitably in location 9 due to chipping defects along the 90 degree edges of the individual prisms.